Self-desiccation of cement-based material refers to the decreasing of relative humidity caused by internal hydration and chemical shrinkage of the cement-based material under enclosed conditions and exchanges with external substances after it is shaped. Under enclosed conditions, the internal relative humidity of the cement-based material will become unsaturated (<100%) from saturated (100%) because of hydration. It is therefore called self-desiccation. Self-desiccation effect is a drive for autogenous shrinkage of the cement-based material. The self-desiccation degree is the basis to calculate autogenous shrinkage of the cement-based material. Autogenous shrinkage is an important reason cause cement-based material to crack in the early stage.
Conventional method to test self-desiccation effect is to test the internal relative humidity of the cement-based material under enclosed conditions by means of the hygrometer. Along with the development of the modern testing technology, the precision and reliability of the hygrometer are continuously increasing. However, the reliable testing range of the hygrometer is below 99% due to limitation of the testing principle and the equilibration time is fairly long. The relative humidity of the cement in early stages is mostly above 99%. It is a world-wide problem to conduct humidity test under such high humid environment. From the typical humidity curve made via a conventional hygrometer, the real humidity cannot be measured for a long time as the hygrometer is still in the equilibration time after initial setting of the cement. Consequently, we have to wait (it is generally 1 day) until the cement-based material becomes hardened if we intend to use the hygrometer. Additionally, we cannot carry out measurement by using the hygrometer during the period the cement-based material is from being shaped after addition of water to 1 day.
The researcher of this invention has tried to use the principle of tensiometer to test the self-desiccation process (the corresponding relative humidity range is 100%-99.98%) from the cement-based material being shaped after addition of water to the pore's negative pressure becoming about 80 kPa. However, it is found out that 80 kPa is only when the cement-based material is finally set. It is unnecessary to test the water consumption and self-desiccation process (pore's negative pressure is at 80 kPa-2000 kPa and corresponding relative humidity range is at 99.98%-99.5%) when the cement-based material is from final set to water being added for 1 day by using the principle of tensiometer.